In connection with the manufacture of a sheet of paper, a first stage consists in converting the paper pulp, by means of the paper machine, into a sheet of paper, which is not yet coated. The paper pulp essentially contains natural or synthetic fibers, water, and one or more mineral fillers such as calcium carbonate, together with various other additives, said mineral filler being mixed with the fibers in the form of an aqueous suspension and/or dispersion. It is then said that a mineral filler (such as calcium carbonate) is used as a mass filler. The sheet thereby obtained can then possibly be coated during a second operation which consists of depositing on the surface of the support paper an aqueous compound called a “paper coating”, which notably contains water, one or more mineral fillers (such as calcium carbonate), one or more binders, and various additives.
Calcium carbonate, of natural or synthetic origin, is commonly used as a mass filler, making it possible to improve the optical properties of the sheet of paper, particularly including its gloss. This calcium carbonate is generally implemented in the form of an aqueous suspension and/or dispersion, wherein the mineral material is stabilized in water by means of dispersing agents.
In the particular case of sheets of paper known as “supercalendered” papers which undergo a very high compression in the calendering operation with a view to providing a very smooth sheet, intended notably for offset and rotogravure printing, it is well known that the gloss of the final product deteriorates as the pH of the medium containing the pulp and the mineral material increases. Attempts have thus been made to manufacture such papers in an acidic environment, through the introduction, just before manufacturing the sheet of paper, of carbon dioxide or a weak acid particularly including phosphoric acid, with a view to reducing the pH of the medium containing the pulp and the mineral material, and to maintaining it at a value of under 7.
However, it is well known that calcium carbonate, used in an acidic medium, becomes partially soluble, leading to the presence of calcium ions in the aqueous phase. This solubilization poses the double problem of:                reducing the quantity of calcium carbonate which the formulator seeks to introduce in the final stage in the sheet of paper, since this carbonate is partly solubilized,        and of increasing the concentration of calcium ions in the aqueous phase of the medium containing the pulp and the calcium carbonate, and thus of increasing the pH of the medium, which harms the final optical properties of the paper, such as its gloss.        
There is thus a real technical need for a solution enabling calcium carbonate to be used in a method for manufacturing paper in an acidic medium, to compensate for the disadvantages outlined above.
This solution was engineered using the technology known as “Weak Acid/Chelatant” (or “WAC”), which was described in the document “Neutral ground wood papers: practical and chemical aspects” (International Paper and Coatings Chemistry Symposium, 5th, Montreal, QC, Canada, Jun. 16-19, 2003, Publisher: Pulp and Paper Technical Association of Canada, Montreal, Quebec). It is based on the use of a weak acid in combination with a chelating agent of calcium. Calcium carbonates manufactured in this manner were then qualified as “acid-stabilized” or “acid-resistant” calcium carbonates. The action mechanism of the “weak acid/chelating” technology, as proposed in this document, is as follows. Initially, the reaction between the calcium carbonate and the weak acid gives rise to chemical species which play a role as a pH buffer, in relation to the medium in which the calcium carbonate is used. For example, when phosphoric acid is used as a weak acid, soluble species are created in the presence of calcium carbonate, namely calcium hydrogeno- and dihydrogenophosphate. These species make it possible to buffer the aqueous medium in which said carbonate is used. A chelating agent is then introduced into the medium, the role of which is to stabilize the previously created buffer system, notably through the mechanism of sequestering the ions in the solution. The Applicant indicates that the expression “chelating agent” refers to a compound having the ability to bind itself to a positive ion with which it forms a compound (or chelate), in which the central atom is bound to the neighboring atoms by at least two links, forming a ring structure (see on this subject the definition given by Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 5, pp 339-68).
Thus, document U.S. Pat. No. 5,043,017 describes an “acid-stabilized” calcium carbonate, by combining a chelating agent of calcium (or a combined base) with a weak acid, the carbonate thereby obtained being in chemical balance with the chelating agent or combined base, and also with the weak acid. The chelating agent is very preferentially sodium hexametaphosphate, while the weak acid is chosen from among the phosphoric, hexametaphosphoric, citric, boric, sulfurous, and acetic acids, or mixtures thereof. A calcium carbonate is thereby obtained which can be used in the manufacture of paper in an acidic medium, giving said paper improved optical properties in terms of its light scattering coefficient, opacity and gloss.
In relation with the previous document, document U.S. Pat. No. 5,156,719 describes a method to increase the optical properties of a paper (said paper being manufactured notably in an acidic medium), such as its gloss, through the use of “acid-stabilized” calcium carbonate, as described in document U.S. Pat. No. 5,043,017.
As for document WO 98/29601, it describes an aqueous suspension of “acid-stabilized” calcium carbonate comprising water, calcium carbonate, and a stabilizing acid found in a sufficient quantity to achieve a pH less than 7. Said stabilizing acid is chosen from among a water-soluble salt of calcium, a weak acid, a chelating agent, and the mixture of a water-soluble salt of calcium with a weak acid or a chelating agent. Said chelating agent is notably chosen from among the polycarboxylic, acrylic, phosphonic or sulfonic acids. A calcium carbonate is thereby obtained which breaks down weakly when the pH is below 7, and which allows the manufacture, in an acidic medium, of a paper with improved gloss.
The presence of a chelating agent of calcium has even been extended to the manufacture of paper in a neutral medium, as indicated in document WO 97/41302. The goal of this document is to limit not only the presence of calcium ions in the papermaking method, but also, more generally, the presence of divalent ions: as instructed by this document, these divalent ions may be derived from the breakdown of calcium carbonate, but also through the presence of certain chemical compounds in the method for manufacturing a sheet of paper, such as aluminum sulfate or polyaluminum chloride. In the method for manufacturing a sheet of paper, these divalent ions may chelate the gluing agents such as rosine, which reduces the effectiveness of the said gluing agents: a reduction of the phenomenon of retention of mineral fillers within the fibers is then observed. With a view to limiting the presence of divalent ions in the method for manufacturing a sheet of paper, document WO 97/41302 proposes a solution which lies in the use of a chelating agent or precipitating agent of these divalent ions, where said agent is a phosphated compound.
However, it now appears that these solutions based on the use of chelating agents of divalent ions, and notably the calcium ion, have a serious disadvantage for the person skilled in the art. Indeed, due to their chelating effect, these agents give rise to complexes of ions present in the aqueous phase, particularly including soluble calcium complexes. When such complexes form, an excessive concentration of these species in an aqueous phase will lead to the precipitation of insoluble species.
However, the formation of insolubles in water used in the method for manufacturing a sheet of paper is particularly undesirable, because it leads to the formation of deposits which can clog sieves, filters, pumps, pipes, or boilers or refrigerators, and such a phenomenon can even lead to the total blockage of the papermaking machine. These problems are related in “Deposition and scaling in the pulp & paper industry” (Congresso e Exposicao Anual de Celulose e Papel, 35th, Sao Paulo, Brazil, Oct. 14-17, 2002 (2002) 516-534 Publisher: Associacao Brasileira Tecnica de Celulose e Papel, Sao Paulo, Brazil) and “Treatment of waste water from a cellulose and paper factory by ozone and activated sludge” (Vom Wasser (1976), 46, 221-39), which note the disadvantages relating to the deposition of calcium oxalate-based insoluble species in papermaking installations, or in the document “Calcium oxalate in bleach plant filtrates” (Minimum Effluent Mills Symposium, San Francisco, Oct. 23-24, 1997 (1997), 51-62 Publisher: TAPPI Press, Atlanta, Ga.) which generalizes the problem of deposits of insoluble chemical species in the papermaking process in the presence of calcium and barium ion complexes.
At this stage of the account, the Applicant wishes to emphasize that, although the state of the technique clearly demonstrates that the problems relating to the presence of calcium oxalate-based insoluble species are well-known to the papermaking industry, there is no document demonstrating that these disadvantages may be related to the use of calcium-chelating agents as used according to the “weak acid/chelating” technology. The identification of this new technical problem is thus in itself one of the merits of which the Applicant has shown evidence.